1. Field of the Invention
The present invention relates to an active matrix display device suitably used as reflective and transflective display devices and the like displaying images by the use of external light reflection.
2. Description of the Related Art
In the field of display devices, active matrix liquid crystal display devices capable of displaying high-quality images have been widely used. This type of liquid crystal display devices includes a liquid crystal layer disposed between a pair of substrates. One of the substrates has pixel electrodes arrayed in a matrix thereon. Each pixel electrode has a switching element using a thin film transistor or thin film diode to ensure switching from one pixel electrode to another. Thus, the size and the resolution of the liquid crystal display devices can be easily increased.
Among these liquid crystal display devices, reflective and transflective display devices have been proposed in which a diffuse-reflective reflecting film is provided on the lower substrate, which is more distant from viewers than the other substrate, to reduce parallaxes, thereby displaying bright images in a wider area.
For example, Japanese Unexamined Patent Application Publication No. 5-281533 discloses a liquid crystal display device including light-reflective pixel electrodes 200 arranged in a matrix when viewed from above, shown in FIG. 20, and many small doughnut-shaped projections 201 and small cylindrical projections 202, shown in FIG. 21, on the pixel electrodes 200. In this liquid crystal display device, the doughnut-shaped projections 201 having a height of about 1 μm, an outer diameter of about 20 μm, and an inner diameter of about 5 μm and the cylindrical projections 202 having a height of about 1 μm and a diameter of about 14 μm are formed on the surface of an insulating layer coating thin film transistors 204. These projections 201 and 202 are covered with the pixel electrodes 200 formed of a conductive film. Thus, the plurality of light-reflective projections 201 and 202 are formed in the above-described shapes.
Also, Japanese Patent No. 3019058 discloses a liquid crystal display device in which thin film transistors 212 are formed on a lower substrate side of a pair of substrates 210 and 211 lying in the vertical direction, as shown in FIG. 22, and covered with an insulating layer 213, and light-reflective pixel electrodes 215 are formed on the insulating layer 213. The insulating layer 213 has projections and depressions, so that the pixel electrodes 215 overlying the insulating layer 213 have at least two types of regions 216 and 217 with different light scattering directivities on the surface thereof. The maximum sizes of the regions 216 and 217 are set to be a predetermined area or less (for example, 5 mm square or less).
In order to form the pixel electrodes 215, the surface of the insulating layer 213 may be subjected to sandblasting to increase its roughness, or the upper surface of glass may be etched with fluorine to increase the roughness. Alternatively, the surface of a polyimide film may be dry-etched to increase the roughness.
In the case shown in FIG. 22, the regions 216 are formed in a shape having gentle slopes and the regions 217 are formed in another shape having steep slopes. Thus, these types of regions 216 and 217 allow light to reflect differently from each other. While the regions 216 produce scattering light having relatively high directivity, the regions 217 produce scattering light having relatively high diffusibility, as designated by arrows in FIG. 22.
A reflecting film formed by the above-mentioned sandblasting or etching reflects light such that the incident angle and the reflection angle are bilaterally symmetrical with respect to the vertical center line between the incident light and the reflected light; hence, the reflection from the reflecting film exhibits Gaussian distributions, as shown by curves A1, B1, and C1 in FIG. 23.
In general, viewers watch a liquid crystal display device with the liquid crystal panel tilted with respect to the viewers. Therefore, while a liquid crystal display device exhibiting a Gaussian distribution in reflection can display bright images having a narrow peak in a specific range, as shown in FIG. 23, brightness is liable to decrease in a light-receiving region close to the viewer side, which is the most important region in practice, from the normal to the liquid crystal panel. Also, sufficient brightness covering a large area cannot, disadvantageously, be provided in the practically most important light-receiving region close to the viewer side from the normal to the liquid crystal panel.
In addition, when sandblasting or etching treatment is performed on the insulating layer, acting as the base layer of the reflecting layer, various types of wires and the thin film transistors or thin film diodes underlying the reflecting layer are likely to be damaged.
Also, the liquid crystal display device having the projections 201 and 202 shown in FIGS. 20 and 21 lead to the same reflection properties as in the foregoing liquid crystal device, having reflection characteristics exhibiting bilaterally symmetrical Gaussian distribution, and, therefore, has the same problem as in the foregoing liquid crystal display device.